The present invention relates in general to control valves, and, more specifically, to resetting a double valve for controlling a single flow of pressurized fluid in response to simultaneous activation of a pair of control switches.
Machine tools of various types operate through a valving system, which interacts with a pneumatically-controlled clutch and/or brake assembly. For safety reasons, the control valves that are used to operate these machine tools require the operator to activate two separate control switches substantially simultaneously to ensure that an operator's hands are away from the moving components of the machine tool when an operating cycle is initiated. Typically, an electronic circuit responsive to the two control switches generates a pilot control signal applied to the pilot valves for switching the main fluid circuit of the valve to control delivery of compressed air (or other fluid) to the machine tool to perform its operating cycle.
Double valves operating in parallel in one valve body have been developed to ensure that a repeat or overrun of a machine tool operating cycle cannot be caused by malfunction of a single valve unit (e.g., a valve becoming stuck in an actuated position). Thus, if one valve unit fails to deactuate at the proper time, the double valve assumes a configuration that diverts the source of compressed air from the machine tool. A double valve is shown, for example, in commonly assigned U.S. Pat. No. 6,478,049 to Bento et al, which is incorporated herein by reference for all purposes.
In addition to providing protection against the repeat or overrun of the machine tool, it is desirable to monitor the double valve for a faulted valve unit and to prevent a new operating cycle of the machine tool from being initiated. Thus, prior art systems have caused the double valve to assume a lock-out configuration when a single valve unit is in a faulted condition so that the double valve cannot again be actuated until it has been intentionally reset to clear the faulted condition.
More specifically, a double valve assembly includes two electromagnetically-controlled pilot valves. Typically, the pilot valves are normally closed. The double valve assembly includes two movable valve units, each with a respective exhaust poppet between the outlet port and the exhaust port of the double valve and a respective inlet poppet between the outlet port and the inlet port of the double valve. When the pilot valves are normally closed, then the exhaust poppets are normally open and the inlet poppets are normally closed. Each of the pilot valves is moved to an actuated position in response to an electrical control signal from a respective operator-controlled switch, which typically causes the exhaust poppets to close and the inlet poppets to open. Any time that 1) a valve unit fails to deactuate properly, 2) a valve unit fails to actuate properly, or 3) the pilot valves are actuated or deactuated non-simultaneously, then at least one valve unit becomes locked in a faulted position where its exhaust poppet cannot be closed (thereby preventing the outlet from becoming pressurized).
In a typical reset operation, one or more faulted movable valve units are returned to their deactuated positions by the application of pressure from a source via a 2- or 3-way control valve to a reset port. The pressure causes a reset piston to extend in a manner that drives or pushes a faulted valve unit out of the faulted position so that the control valve can be actuated to start an operating cycle of the machine tool. If a faulted valve unit is present, then it is desirable for a machine operator to service the control valve to repair the valve unit. However, operators may sometimes attempt to maintain the reset function continuously in order to continue machine tool operations without repairing a faulted valve unit by “tying down” the reset control valve. It is desirable to provide an anti-tiedown function in the design of the double valve so that the tying down of the reset valve cannot prevent the control valve from locking out in response to a faulted valve unit.
Prior anti-tiedown mechanisms have required added components that were relatively complicated and that added significant cost to the valves. Furthermore, in some valve configurations, it has been possible to tie down the main pilot switches and operate a machine tool using the reset control as a single control point (thereby defeating the mechanism for requiring simultaneous activation of the two manual switches). Prior anti-tiedown mechanisms have not prevented this type of operation of the control valve.